Support joining snippets by Hub username

This is in anticipation of importing snippet data into _data/public, whereby
the import script will replace the email address in the "Username" field with
the corresponding Hub username.
Minor plugin tweaks to support _data/public
Script to import stripped data into _data/public
Remove 18F/data-public submodule

This is in preparation for checking a scrubbed copy of 18F/data-private files
into the Hub repo directly.
Import data into _data/public via import-public.rb
Update README to reflect new build instructions

The submodules are no longer required to build a running instance of the Hub.
Merge pull request #25 from 18F/replace-data-public

Replace _data/public submodule with import